1. Field of the Invention
The present invention relates to a mobile communication system and a method of processing a handoff for the same.
2. Background of the Related Art
A CDMA mobile communication system includes mobile stations for moving around unspecified regions, base stations for reception/transmission of messages from/to the mobile stations and managing radio resources, a control station for controlling the base stations and reception/transmission of messages from/to the base stations, and a mobile switching center for connection to the control station, mobile switching centers of other communication systems or other communication networks, for communication between mobile stations or between the mobile station and a fixed station to facilitate communication anytime, anywhere, and with anybody within the communication network.
One feature of the mobile communication system is the assurance of maintaining communication in view of the mobility of the subscriber's mobile station. One method of supporting this feature is hand off technology. The hand off technology is a technology for switching a communication path between the mobile station and the base station so that communication can be continuous, even as the mobile station moves in relation to the base station that provides the mobile station with a service, or a service region of a sector.
Typically, there are three types of hand off: a soft hand off, a softer hand off, and a hard hand off. The soft hand off occurs when the mobile station moves between adjacent base stations while a frequency and a frame offset are maintained. In soft hand off, the mobile station sets up another communication path between cells of a destination base station, while the present communication path between the mobile station and the base station is maintained for a time period, to transmit a voice packet and/or a data packet on many channels. The present communication path is dropped when the mobile station moves to a destination cell where maintenance of the present communication channel is no longer required. The softer hand off occurs when the mobile station moves between sectors covered by the same base station, while a frequency and a frame offset are maintained wherein the mobile station sets up a new communication path with a destination. In softer hand off, the present communication path between the mobile station and the base station is maintained, in order to support the transmission of a voice packet and/or a data packet on many channels. The present communication path is dropped when the mobile station approaches the destination sector within a sufficient distance such that maintenance of the present communication path is not required anymore.
The hard hand off occurs if the frame offset is changed, or the used frequency group is changed. Another instance of hard hand off occurs when the mobile station moves to an adjacent base station, or the mobile station moves to another mobile switching center, wherein unlike the softer hand off or the soft hand off, the mobile station can not maintain the present communication path, but a newly set up communication path is used in transmission of the voice packet and/or data packer.
The softer or soft handoff in the CDMA mobile communication system in which both base stations use the same frequency proceeds smoothly without dropping calls, since no ping-pong of the hand off has occurred. Ping-pong is a term of art used to characterize a repeated hand off process. However, the softer or soft hand off cannot be used between systems using different, respective frequency groups because the ping-pong of hand off of the different frequencies leads to a very high call drop ratio. In order to process the hand off between different system provider regions, it is desired that each of the base stations of the systems is provided with a pilot beacon for giving information on a hand off starting time point to the base stations and the mobile station, respectively.
FIG. 1 illustrates a region between cells in a related art CDMA mobile communication system. FIG. 1 shows the information on a hand off starting time point to the mobile station, in case the mobile station moves from a cell ‘A’ to a cell ‘B’, the pilot beacon B provided to the cell ‘B’ area transmits a pilot signal of a frequency used in the cell ‘A’ at a strength the same with a signal strength at a boundary area of the cell ‘B’ for making hand off from the cell ‘A’ to the cell ‘B’. In case the mobile station moves from a cell ‘B’ to a cell ‘A’, the pilot beacon A provided to the cell ‘A’ area transmits a pilot signal of a frequency used in the cell ‘B’ at a strength the same with a signal strength at a boundary area of the cell ‘A’ for making hand off from the cell ‘B’ to the cell ‘A’.
However, the movement causes the ping-pong of hand off in which a call add/drop occurs repeatedly, due to the ceaseless variation of the signal strengths of frequencies of the cells in the boundary area in hand off process, which causes a drop of a call in the boundary area of systems.
More particularly, a related art handoff process is performed using the pilot beacons A and B as shown in FIG. 1. When a calling mobile station (or terminal) is communicating with a called mobile station as it is moving from the cell A to the cell B, the calling mobile station may estimate a strength of the pilot signal having the same frequency as in the cell A, the pilot signal is sent by the pilot beacon B.
Specifically, if the signal strength of the pilot signal meets the handoff condition, or criteria, the calling mobile station (MS) transmits a Pilot signal Strength Measurement Message (PSMM) to the base station A. The base station in cell A analyzes the message, and performs an inter-frequency hard handoff of a call from the cell A to the cell B, so that the calling mobile station may communicate with the called mobile station through the cell B. However, in the boundary area of the two cells A and B, strengths of pilot signals received from the two cells A and B are continuously varied, and thus the inter-frequency hard handoff of the call from the cell B to the cell A may be performed again.
As mentioned earlier, the repeated handoff process is called the ping-pong phenomenon. During the ping-pong process, the strength of pilot signals meets the handoff condition, but the call is disconnected since a signal strength of a forward traffic channel or a reverse traffic channel is weak. Also, the respective pilot beacons should transmit the pilot signals for all the frequency groups used by different network provider regions.
The related art technology as described above has the following disadvantages. First, since the hard handoff is a type of inter-frequency hard handoff, the call drop is frequently produced in the boundary area of the two different network provider regions due to the ping-pong phenomenon and other factors influencing hand off.
Second, due to the frequent production or additional occurrence of the ping-pong phenomenon, an additional load is produced with regard to the handoff-related message, and the load causes the communication quality for the call to deteriorate.
Third, the respective pilot beacons should transmit the pilot signals for all the frequency groups used by the different network provider regions and is a costly solution.